code: def testFind_gradient(self):
      mydlm2 = deepcopy(self.mydlm)
      self.mydlm.fitForwardFilter()
      mydlm2.fitForwardFilter()
      mse0 = mydlm2._getMSE()
      mydlm2._setDiscounts(
          list(
              map(
                  lambda x: x + self.mytuner.err,
                  self.mydlm._getDiscounts(),
              )
          )
      )
      mydlm2.fitForwardFilter()
      mse1 = mydlm2._getMSE()
      expect_gradient = (mse1 - mse0) / self.mytuner.err
      self.assertAlmostEqual(
          expect_gradient,
          self.mytuner.find_gradient(
              self.mydlm._getDiscounts(), self.mydlm
          ),
      )
  
file_path: pydlm/tests/tuner/testDlmTuner.py
imports: import unittest
  import numpy
  from copy import deepcopy
  from pydlm.tuner.dlmTuner import modelTuner
  from pydlm.modeler.trends import trend
  from pydlm.dlm import dlm
label_desc: Use image node_id 2 for calling the testModelTuner obj's underlying member method code with example usage: obj.testFind_gradient() without return types